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SEED DEVELOPMENT AND MATURATION
Howard C. Potts 1
The culmination of the life cycle of most higher plants is the
development of its reproductive unit, the seed . The seed, in terms of
sexual reproduction , is a mature, fertilized ovule consisting of the
embryonic axis, food reserves and an outer covering .
The life of any seed can be divided into four stages : ( 1) 1 ts
origin in the flower of the mother plant, (2) its development and
maturation , (3) its resting stage , and (4) its resumption of growth or
germination.
Seed Development and Morphology
At a specific time in the life cycle of all flowering plants,
the physiological and biological processes change from the production of
vegetative organs - leaves & stems - to the reproductive organs - the
flowers.
Des pite differences in the appearances of flowers, those
floral organs involved in the formation of seed are quite similar for
most species . The male and female reproductive organs of most flowering
plants are comparable. Typically, each flower is produced at the end of
a stalk , the pedicle, which is :nodified and specialized for reproduction. The tip of the pedicle , where the floral organs are attached, is
usually somewhat enlarged . This enlarged region is called the Receptacle .
I n a typical complete flower, the receptacle gives rise to the
four basic floral organs - sepals, petals , stamens, and pistil.
The sepals and petals have no direct role in actual seed devel opment, but serve as a protective organ and in some cases attract
insects, necessary for pollination. The Stamens, the male reproductive
organs of a flower, are located inside the petals.
Each stamen is
typically composed of a threadlike stalk , the Filament, which is
terminated by an enlarged four - lobed, usually yellow organ - the Anther.
The primary r ole of the filament is to position the anther so the pollen
is dispersed in a manner typical of its species. The pollen grains are
formed inside the anther. When mature, the anther ruptures and releases
the pollen.
1Agronomist, Seed Technology Laboratory,
State , MS.
Miss. State University,
Miss .
66
The femal'? reproducti V"=! organ , the
center of the flower .
The enlarged base ,
surrounded by the receptacle, is the Ovary.
the stalk-like Style , which terminates-m-an
Stigma (Figure 1).
Pistil, is located at the
which may rest on or be
Extending from the ovary is
expanded portion called the
Pistils and ovaries can be simple or compound.
Within each
ovary one or several ovules can be produced .
The common bean is an
example of a species with a simple pistil that produces several ovules
(Figure 2). All species of the grass family have a simple ovary whj ch
produce only one ovule.
A compound ovary is composed of two or more
cavities or carpels . One or many ovules may be formed inside each
cavity.
Cotton and okra are examples of species having compound
ovaries.
Some species produce flowers having no stamens or non- functioning ones . Such flowers are referred to as pistillate or male- sterile.
The flowers of the female inflorescence or ear of maize have-no stamen.
When the pistil is absent, the flower or floret is referred to as
staminate .
The florets produced on the tassels of maize are normally
staminate . Flowers and florets containing neither male nor f>?male reproductive organa or containing non- functioning ones are at"=!rile and , of
course , are not involved in seed formation.
Some species , such as maize and cucumber , produce staminate
flowers on one part or the plant and pistillate flowers on another part
of the same plant . Such plants are referred to as being Monoecious . In
a few species, individual plants produce only pistillate or staminate
flowers . Such species are Dioecious. Papaya and date palm are examples
of dioecious species .
Grass flowers are so small and inconspicuous that many people
think grasses have no flowers .
Because they are so different from
flowers of most other plants, a aeparat"=! terminology is applied to some
of their floral organs. The complete perfect flower of a grass plant
consists of a pistil with a single, simple ovary, two styles with
featherlike stigmas , and three stamens (rice has six stamens). The
reproductive organs of grasses are enclosed by two leaflike structur<?S.
The larger structure is the Lemma and the smaller the Palea.
The
Lodicules, two small saclike structures located inside the l~and the
palea, expand when the flower's reproductive organa are mature. Expansion of the lodioules causes the lemma and palea to separate and expose
the stamens and stigmas . The lemma , palea , and enclosed organs are
called a floret. Florets are produced individually or along a central
axis in groups of two or more; they are connected by small stems , known
as Rachillas . The individual or groups of flor<?ts ar~ subtended by two
leaflike structures called glumes (Figure 3) .
The maize flower is a significant exception to the basic organi zation of grass florets . The female florets are arranged in pair~d rows
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....,..,___ Stigma
Stamen
.,.___ _ Style
Filament
Pistil
Ovary
Petal
Receptacle
Figure 1. Diagram of a longitudinal section of a complete flower.
O vary wall
Figure 2. Comparison of simple pistil of the common bean, Phaseolus vulgaris (A),
and sesame, Sesamum indicum (8 ), having a compound ovary.
68
Lemma
A
Figure 3. Open oat floret, Auena satiua (A) and rice spikelet, Oryza sativa lB\.
Fiicment
Pollen sac
' ''
\
I
''
I
'
.
:-:-:or:-;er
f
,.
c e ~1
/
/
/
/
/
/
/
/
Figure 4. Development of a pollen grain from a microspore mother ceiL
69
along a c~ntral axis, the cob, and each floret produces a long single
stigma- style (the silk) .
Only the essential parts of a flower - the pistil and stamens are directly involved in seed formation.
The process of pollen grain
development is called microsporogenesis. A cross section of a developing anther reveals that each of the four lobes is filled with cells
called microspore mother cells (Figure 4).
Through meiosis, a c~ll
division process by which the number of chromosomes is reduced, each
microspore mother cell divides t wice .
These divisions result in the
formation of four microspor es, each containing half (1N) the chromosome
number of the mother plant . · The wall of the m1 crospore becomes tr.e wall
of the pollen grain. Prior to being shed , this wall thickens and the
outer surface usually becomes roughened with spines, pits, plates , or
ridges, according to the species .
The ovules develop from cells of the placenta whi ch line the
inside of the inside of the ovary wall . Each oVUle starts as a mass of
cells, called the nucellus , whi ch enlarges rapidly . This tissue is one
to several cell layers in thickness and surrounds the single but much
larger megaspore mother cell . The developing ovule is raised from the
ovary wall on a short stalk called the Funiculus.
A.s the funiculus
elongates, one or more layers of cells, the Integuments, envelope the
megaspore mother cell , except over a small opening called the Micropyle .
The functional megaspore then divides mitotically three times , giving
rise to an embryo sac containing eight genetically identi cal nuclei.
In the mature embryo sac the large cell near the micropyle is
the Egg cell. The smaller cells at each side of the egg cell are called
Synergids.
The two nuclei near the center are the polar nuclei. The
three cells at the end opposite the egg cell are the Antipodals . Thus , a
mature ovule ready for fertilization consists of an embryo sac with
generally six cells and two polar nuclei . The embryo sac is surrounded
by the nucleus, which , except for the micropyle, is surrounded by the
integuments . The entire structure is attached to the ovary wall by the
funiculus (Figure 5).
When a pollen grain lands on a sti~a of the same species ,
pollination has occurred . Mature pollen grains may be transferred from
the anther to the stigma by gravity, wind , or various insects .
\-/hen
pollen is transferred to the stigma of flowers or the same plant ,
self- pollination occurs .
When pollen is transferred to stigmas in
gP.netically different plants , cross- pollination takes place .
Events occurring after pollination are similar for all flowP.ring
plants. Normally, thP. pollen grain germinates within a few hours after
contacting the stigma, producing a pollen tube which grows through thP.
style and the ovary wall. The tip of the pollen tube passes through the
micropylP. and penetrates the embryo sac where the pollen tube ruptures,
discharging its two sperm nuclei.
One of the sper:n nuclei joins with
70
Megaspore morher cell
Ovary wail
B
Nucellus
'
'
I'nteguments
'
''
Func:ior:ai megaspore
Cegenerari:1g
megaspores
Nuceilus
c
0
Figure 5 .
Di~~m
of embryo sac development.
71
and fertilizes the egg cell, forming the first cell of the new plant,
the Zygote.
The second sperm nucleus fuses with both polar nuclei to
form the primary endosperm nucleus.
Fertilization of the egg cell
re- establishes the normal (2N) chromosome number of the species .
The
fusing of the three nuclei initiates the formation of the endosperm
(3N) .
The two separate unions, sperm with egg and sperm with polar
nuclei, are referred to as double fertilization, an event unique to the
plant kingdom (Figure 6) . Double fertilization must occur within every
ovule in the ovary or the seed will not be formed.
In beans , for example, this means that, depending on the number
of ovules , one or more pollen grains are rP.quired to fertil ize the
ovules produced in each ovary.
In rice, only one pollen grain is
necessary for fertilizati on because only one ovulP. is formed in each
pistil.
Seed Development
Following fertilizat.ion, the newly formed cells, the zygote and
endosperm , start dividing.
The zygote produces a row of cells, the
proembryo (Figure 7).
After a few hours or days , the proembryo cell
farthest from the micropyle enlarges and divides, forming the first
cells of the embryo.
Of the cells formed by this division , the cell
nearest the micropyle gives rise to the roots and associated underground
plant parts.
The other cell gives rise to the above- ground parts:
stems, leaves, and eventually flowers (Figure 8) .
A few days after fertili zation the first difference becomes
ap'p arent between embryos which will have two cotyledons ( dicots) left
and those which will have only one (monocots) right (Figure 9) . As the
seed continues its development, it increases in size and dry weight
.until growth is completed and germination capability is achievP.d . This
point is generally referred to as physiological maturity of the seed.
No further morphological development takes place.
The mature embryo of most dicotyledonous seed consists of an
embryonic axis to which the two cotyledons are attached. The formation
of these two structures utilizes most or all of the endosperm, depending
on the species. At one end of the axis , above the cotyledonary node is
the Epicotyl or Plumule which will produce the above- ground structures
of the plant.
At the other end of the embryonic axis is the Radicle
which will develop into the primary root. Between the radicle and the
cotyledonary nodes is an area called the Hypocotyl. The embryonic axis ,
cotyledons and endosperm (when not totally consumed in embryo formation)
are completely covered by the Testa (seed coat) which is formed by
drying and hardening of the integuments. The scar on tr.e surface of the
testa, the Hilum , is formed when the funiculus is broken from the now
mature , fertilized ovule (seed ) (Figure 10). The micropyle can be seen
on the testa of some seeds .
Examples of dico ts 1 ncludes beans and
soybeans .
7'2
........-· Germinated pollen gra.ins
Embryo sac
A
Egg cell
Figure 6. Double fertilization process.
·-.
Figure 7. Development of th': proembryo.
c
73
Figure 8. Relation between polarity of proembryo cells and the future seedling.
' '
.. :e,.: vr:o us
.·~ : · ~ : ~ ,,..:a :-:
- ..
--::-
0
..
Figure 13. Differences between dicotyledonous and monoc01yledonous embryos.
74
Figure 10.
Features of the mature embryo of
a dicotyledonous plane.
Cotylecon
8
Pericarp
Figure 11.
longitudinal section of
a caryopsis. ,
Endosperm
Coryiedon or scutellum
Coleopriie
P lumui~
Semir.al roots
Radic!e
C0leorhiza
75
The development pattern of the monocot seed is very similar to
that of a dicot seed except for the number of cotyledons. Also, in most
economically important monocot species , the P.ndosperm is not totally
consumed during development of the embryo.
Therefore, the mature
monocot seed contains both cotyledonary and endospermic forms of stored
food . Maize, wheat, and barley are examples of monocot seeds .
Seeds of the grass family are classed botanically as fruits. The
embryo , as defined previously, is embedded in the endosperm. The ovary
wal l, or pericarp, rather than the intP.guments , functions as a protect! ve layer outside the seed coat.
This type of one- seeded fruit is
called a Caryopsis.
A longitudinal section of the embryonic axis
reveals those structures common to all seed .
The cotyledon in a
caryopsis is called the Scutellum.
In addition, there are protective
tissues; the Coleoptile encloses the epicotyl, and the Coleorhiza
encloses the radicle. Rudimentary seminal roots , located in the region
of the cotyledonary node , can be seen in most grass species (Figure 11 ) .
The
Mesocotyl is .the region located between the epicotyl and the
cotyledonary node.
It should be evident that for a seed to be valuable for reproductive purposes, its embr yo must be alive . The longevity of any seed
depends on many factors - the environment in which 1 t is stored, its
chemical composition , its physical structure, etc . It is important that
we know the structure of the seed with which we work, not only for
proper selection and use of equipment for harvesting , threshing , and
handling seed but also to relate this structure to different stages of a
quality control program . In purity tests, for example , it is necessary
to know the seed characteristics of the species with which we are
working t o be able to separate it from the other seeds , weeds or inert
matter. It is necessary to know the embryonic structure and its relation
to the tissues of the future plants to evaluate results of germination ,
tetrazolium and other growth tests .
Summary
In summary, the four basic floral organs are sepals , petals,
stamens , and pistil; the latter two are directly involved in seed
formation. Pollen formation takes place in the anthers . When a pollen
grain lands on a stigma of the same species , pollination has occurred.
Then , a sperm nucleus fer tilizes the egg cell, re- establishing the
normal (2N) chromosomes number of the species . The seed or reproductive
unit is a mature fertilized ovule comprised of the plumule, radicle , one
or two cotyledons , food reserves , and an outer covering , the perlcarp or
testa.